NAME
String::Compare::ConstantTime - Timing side-channel protected string
compare
SYNOPSIS
use String::Compare::ConstantTime;
if (String::Compare::ConstantTime::equals($secret_data, $user_supplied_data)) {
## The strings are eq
}
An example with HMACs:
use String::Compare::ConstantTime;
use Digest::HMAC_SHA1; ## or whatever
my $hmac_ctx = Digest::HMAC_SHA1->new($key);
$hmac_ctx->add($data);
my $digest = $hmac_ctx->digest;
if (String::Compare::ConstantTime::equals($digest, $candidate_digest)) {
## The candidate digest is valid
}
DESCRIPTION
This module provides one function, "equals" (not exported by default).
You should pass this function two strings of the same length. Just like
perl's "eq", it will return true if they are string-wise identical and
false otherwise. However, comparing any two differing strings of the
same length will take a fixed amount of time. If the lengths of the
strings are different, "equals" will return false right away.
TIMING SIDE-CHANNEL
Some programs take different amounts of time to run depending on the
input values provided to them. Untrusted parties can sometimes learn
information you might not want them to know by measuring this time. This
is called a "timing side-channel".
Most routines that compare strings (like perl's "eq" and "cmp" and C's
"strcmp" and "memcmp") start scanning from the start of the strings and
terminate as soon as they determine the strings won't match. This is
good for efficiency but bad because it opens a timing side-channel. If
one of the strings being compared is a secret and the other is
controlled by some untrusted party, it is sometimes possible for this
untrusted party to learn the secret using a timing side-channel.
If the lengths of the strings are different, because "equals" returns
false right away the size of the secret string may be leaked (but not
its contents).
HMAC
HMACs are "Message Authentication Codes" built on top of cryptographic
hashes. The HMAC algorithm produces digests that are included along with
a message in order to verify that whoever created the message knows a
particular secret password, and that this message hasn't been tampered
with since.
To verify a candidate digest included with a message, you re-compute the
digest using the message and the secret password. If this computed
digest is is the same as the candidate digest then the message is
considered authenticated.
A common side-channel attack against services that verify unlimited
numbers of messages automatically is to create a forged message and then
just send some random junk as the candidate digest. Continue sending
this message and junk digests that vary by the first character. Repeat
many times. If you find a particular digest that statistically takes a
longer time to be rejected than the other digests, it is probably
because this particular digest has the first character correct and the
service's final string comparison is running slightly longer.
At this point, you keep this first character fixed and start varying the
second character until it is solved. Repeat until all the characters are
solved or until the amount of remaining possibilities are so small you
can brute force it. At this point, your candidate digest is considered
valid and you have forged a message.
Note that this particular attack doesn't allow the attacker to recover
the secret input key to the HMAC but nevertheless can produce a valid
digest for any message given enough time because the service that
validates the HMAC is acting as an "oracle".
NOTE: Although this module protects against a common attack against
applications that store state in browser cookies, it is in no way an
endorsement of this practise.
LOCK-PICKING ANALOGY
Pin tumbler locks are susceptible to being picked in a similar way to an
attacker forging HMAC digests using a timing side-channel.
The traditional way to pick cheap pin tumbler locks is to apply torque
to the lock cylinder so that the pins are pressed against the cylinder.
However, because of slight manufacturing discrepancies one particular
pin will be the widest by a slight margin and will be pressed against
the cylinder tighter than the others (the cheaper the lock, the higher
the manufacturing tolerances). The attacker lifts this pin until the
cylinder gives a little bit, indicating that this pin has been solved
and the next widest pin is now the one being pressed against the
cylinder the tightest. This process is repeated until all the pins are
solved and the lock opens.
Just like an attacker trying to solve HMAC digests can work on one
character at a time, a lock pick can work on each pin in isolation. To
protect against this, quality locks force all pins to be fixed into
place before the cylinder rotation can be attempted, just as secure HMAC
verifiers force attackers to guess the entire digest on each attempt.
SEE ALSO
The String-Compare-ConstantTime github repo
Authen::Passphrase has a good section on side-channel cryptanalysis such
as it pertains to password storage (mostly, it doesn't).
The famous TENEX password bug
Example of a timing bug
QSCAN
Practical limits of the timing side channel
NaCl: Crypto library designed to prevent side channel attacks
AUTHOR
Doug Hoyte, ""
COPYRIGHT & LICENSE
Copyright 2012-2018 Doug Hoyte.
Contributions from Paul Cochrane.
This module is licensed under the same terms as perl itself.